DESCRIPTION (Verbatim from the Applicant's Abstract): This research investigates how steroid hormones affect neuronal structure, function and survival and how these effects influence neural circuits for behavior. In insects, steroid hormones (ecdysteroids) regulate neural changes during metamorphosis, including programmed cell death (PCD) and the growth or regression of dendritic arbors. Steroids exert similar strikingly effects in vertebrate nervous systems, and the evolutionary conservation of steroid hormone receptors and PCD pathways suggests that some molecular mechanisms are shared. In the moth, Manduca sexta, steroid-mediated dendritic remodeling and PCD can be studied in vivo and in vitro at the level of individually identified neurons with known behavioral roles. A segmentally repeated motoneuron designated APR innervates a retractor muscle of the proleg (abdominal appendage) during the larval stage. At pupation, a rise in 20-hydroxyecdysone (20E) triggers proleg degeneration, regression of APR's dendrites and the PCD of APRs in some segments. PCD is a direct response to 20E, regulated by each APR's intrinsic segmental identity. APRs that survive grow new dendrites, are respecified for new pupal functions, and undergo PCD after adult emergence. Electrophysiological, neuroanatomical and molecular biological methods will be applied toward the following objectives. Specific Aim 1 is to test the hypothesis derived from previous electrophysiological studies that the 20E-mediated regression of APR dendrites decreases the number of synaptic contacts with presynaptic sensory neurons in vivo. Specific Aim 2 is to test the hypotheses that the 20E-induced PCD of APRs involves early changes in one or more membrane currents, and that elevated external K+ will block PCD of APRs. Specific Aim 3 is to use cell culture to test the hypothesis that 20E acts directly on APRs to trigger dendritic regression. Specific Aim 4 is to develop a coculture system in which the effects of 20E on synaptic transmission between sensory neurons and APRs can be studied in vitro. Specific Aim 5 is to test hypotheses about the involvement of specific genes and proteins involved in the 20E-induced PCD of APRs, and the regulation of this process by intrinsic segmental identity. These studies will contribute to the understanding of fundamental mechanisms by which steroid hormones control neuronal phenotype and survival, with the long-term goal of improving human health.